1. Field of the Invention
The present invention relates to a hydrogenation catalyst which can be obtained by carrying out essentially the following steps:
(a) treatment of a platinum metal salt with finely divided sulfur, followed by PA1 (b) reduction of the platinum metal salt to metallic platinum metal. PA1 platinum(IV) compounds such as hexachloroplatinic acid and alkali metal and ammonium salts thereof, tetrachloroplatinate or tetrachlorodihydroxyplatinic acid; platinum(II) compounds such as tetrachloroplatinic acid and alkali metal salts thereof or platinum(II) chloride; PA1 palladium(II) compounds such as hexachloropalladous acid and salts thereof or palladium(II) chloride.
The invention also relates to a process for the preparation of the hydrogenation catalyst of the invention, to its use as hydrogenation catalyst particularly for the manufacture of hydroxylammonium salts, and to a process for the regeneration of hydrogenation catalysts.
2. Description of the Related Art
The expense incurred in the preparation and regeneration of a catalyst and the activity, selectivity, and useful life of the catalyst used constitute a decisive factor in determining whether a catalyzed reaction is feasible for industrial use or not. This is the case, for example, when effecting catalytic reduction of nitrogen monoxide with hydrogen to form hydroxylammonium salts. Hydrogenation catalysts containing platinum metal are normally used in this process.
Processes for the preparation of hydroxylammonium salts are known, for example, from DE-PS 956,038 and DE-A 4,022,851. The catalysts used in said references are obtained by precipitation of platinum on to suspended graphite supports if desired with the addition of poisoning means such as compounds of sulfur, selenium, arsenic, or tellurium. The drawbacks of these processes are, on the one hand, the unduly rapid loss of activity and selectivity, and on the other hand, the use of an expensive manufacturing process having a deleterious effect on the environment.
Thus it has hitherto been usual to buffer the reaction mixture containing a platinum metal salt with, say, sodium acetate prior to the addition of the catalyst poison. The buffer is contaminated with organic substances after the reaction has taken place and this must be removed from the waste water, which incurs high disposal costs. Furthermore, buffering is expensive, particularly when use is made of sodium thionite as donor for the sulfur acting as catalyst poison, due to the fact that it is necessary to set up a redox potential.
The sodium thionite mentioned in DE-PS 956,038 is also unstable in atmospheric oxygen, and it must be handled in compliance with certain safety precautions (avoidance of heating above 50.degree. C. and the occurrence of flying sparks, storage in a cool and dry atmosphere).
Other sulfur donors as yet proposed for partial poisoning of the catalyst have equally decisive drawbacks: thiourea is considered to be carcinogenic, sodium thiosulfate and sodium sulfite are decomposable, sodium sulfide is considered to be toxic and moreover, these poisoning agents are not conducive to satisfactory catalyst properties.